During subsea drilling operations it is necessary to remove drilling cuttings from the subsea wellbore to the surface. A marine riser provides a conduit that extends the subsea wellbore to a surface drilling facility.
To preserve the structural integrity of the riser and its connection to the wellbore it is necessary to restrict the riser's exposure to movement due to ocean heave. Slip joints are expansion and contraction tools that accept the movement associated with ocean heave and temperature or pressure changes without allowing the movement to affect the riser pipe on the seafloor. One end of the slip joint is attached to the surface drilling facility and permits ocean heave (vertical motion) while the other end of the slip joint is connected to the riser. As the drilling facility heaves, the slip joint telescopes in or out by the same amount so that the riser below the slip joint is relatively unaffected by ocean heave.
During offshore drilling, fluids are used to aid the drilling operation and oil contaminated mud is produced. The main functions of drilling fluids and contaminated mud include providing hydrostatic pressure to prevent formation fluids from entering the wellbore, keeping the drill bit cool and clean during drilling, and providing an upward pressure to remove the drill cuttings from the wellbore through the connected riser and slip joint conduit to the surface drilling facility.
However, the drill fluids and contaminated mud can be harmful if released into the environment and marine ecosystems. Prevention of leakage of drilling fluids and contaminated mud at the connection between the riser and slip joint is a major concern for the oil industry.
Oil rig owners, drilling contractors and offshore operators want to avoid spillage to prevent loss of hydrostatic pressure in the wellbore and avoid unnecessary costs. Prevention of leakages is also important due to the potential negative impact on the company from an attentive global community focussed on environmental protection.
Currently, the means of containment of the drilling fluids and contaminated mud between the riser and the slip joint is by using packer devices which form part of the slip joint apparatus. Packer devices employ flexible elements which expand externally to seal any gaps between the riser and the slip joint. Typically they operate by pumping a fluid into the flexible element to inflate the packer. Once inflated the packer seals the connection between the riser and slip joint and contains the fluids and mud.
In order to provide complete containment of drilling fluids and contaminated mud when using inflatable packers, it has become common practice to apply high inflating pressure to the sealing packer. However such a high pressure may lead to premature wear of the sealing packer and may result in failure of the packer and leakage of the drilling fluid and mud. Therefore the leakage of drilling fluid will occur when a packer fails or the pressure system fails and is not able to energise the packer.
Currently, riser slip joint systems have a primary upper packer which is energised to seal the overlap connection between the riser and the slip joint and secondary lower packer which is energised in the event of a failure of the primary upper packer to maintain the seal.
WO 2009/086323 discloses a pressure circuit for a riser slip joint system that recognises failure of the upper packer due to hose failure, leakage of drilling fluids or rig air pressure loss. The pressure circuit uses a differential pressure valve which receives the pressure from two points in the circuit, a first measurement point which is closer to the pressure source than the second measurement point. If the pressure at the second point is an operational amount lower than the first point the differential pressure valve switches to de-energise the upper packer and energise a lower packer.
A disadvantage of the system of WO 2009/086323 is that the differential pressure valve is a mechanical switch which may be slow to react. Any delay in response to the upper packer failure means the slip joint may not be sealed and the fluids may leak into the environment. Another disadvantage is that contaminants frequently build up in packer energising systems. The differential pressure valve may become blocked or restricted which could impede or prevent the differential pressure valve from switching between the upper and lower packer. This can lead to critical failure of the containment system. If the upper packer fails and the differential pressure valve is completely blocked with contaminants such that the valve is unable to switch to the lower packer pathway, the lower packer may not be energised and the drilling fluids may not be contained. If the upper packer fails and the differential pressure valve is partially blocked with contaminants and unable to completely switch to the lower packer pathway, the lower packer may not be fully energised, the seal may be inadequate and the drilling fluids may not be fully contained.
Systems that use pressure transducers to monitor the fluid pressure supplied to a primary upper packer have also been proposed. Such systems use a combination of a pressure transducer and solenoid valves. If a pressure decrease is detected in a fluid supplied to a primary upper packer, the supply to the primary upper packer is closed and a secondary lower packer is activated. However, these systems have limited functionality as they are restricted to switching between the activation of the upper packer or the lower packer.
It is the object of the present invention to obviate or at least mitigate the foregoing disadvantages of packer management systems.
It is another object of an aspect of the present invention to provide a packer activation system that is capable of facilitating control of packer activation and ensuring that the seal integrity between the riser and the slip joint is maintained.
It is a further object of an aspect of the present invention to provide a packer activation system which reliably activates the packers. Further aims of the invention will become apparent from the following description.